In order to test communication and other electronic equipment, noise generators are frequently utilized. The most common type of noise source is an analog device which relies upon a thermal noise diode to generate Gaussian noise.
In particular, commercial noise sources generally depend on the statistics of electron flow across PN junctions to generate noise which has a Gaussian amplitude distribution and a flat frequency spectrum. Generally, the noise power level is known only approximately and it will vary with time and the ambient temperature. The testing of electronic equipment at high bit rates typically requires a wide band noise source. For some testing applications, e.g., to match a lower data rate, the noise can be filtered to reduce its bandwidth. However, this results in a concomitant reduction in the amplitude of the noise, thereby requiring amplification to restore the noise to its original level. Thus, for some testing applications a number of analog noise generators, each providing a different bandwidth, are utilized to cover the desired bit rate range. However, such an approach is not without some drawbacks, e.g., precise amplification of the various generators must be achieved.
The generation of noise via the use of a digital source has been proposed as an alternative to analog noise generation. In this connection pseudorandom binary sequence generators, e.g., shift registers, have been used as noise sources in commercial instruments for some time. Typically, analog noise is generated from the binary output of such registers by severely limiting its bandwidth with an analog low-pass filter. Alternatively, it has been proposed (See Lipson, Foster and Walsh, "A Versatile Pseudorandom Noise Generator," IEEE Trans. Instrumentation and Measurement, Vol. 25, No. 2, June 1976) to use a weighted sum of the binary levels at various points on a shift register to synthesize a digital signal. This approach generates an approximately Gaussian amplitude distribution while also flattening the output frequency spectrum. However, the resultant noise bandwidth is severely limited, e.g., is 1/20 of the clock frequency at which the shift register is shifted.
Another approach to the synthesis of noise via digital techniques is to utilize a digital filter to generate a Gaussian amplitude distribution, but with the same (sine X)/X bandwidth distribution as the input sequence. (See Rowe and Kerr, "A Broad-Spectrum Pseudorandom Gaussian Noise Generator", IEEE Trans. Automatic Control, Vol. AC-15, No. 5, October 1970). Accordingly, this approach does not meet the requirement for flat noise spectrum.
The concept of separating the function of generating a flat frequency response from that of generating a Gaussian amplitude response has been discussed (See, Neuvo and Ku, "Analysis and Digital Realization of A Pseudorandom Gaussian and Impulsive Noise Source", IEEE Trans. on Communications, Vol. COM-23, No. 9, Sept. 1975). However, this approach has not been applied to real-time generation of wide band noise.
Still another approach to digital synthesis of noise ha been proposed. That approach utilizes plural digitally generated noise samples for generating an analog output by means of a digital-to-analog converter. (See Kafadar, "Gaussian WhiteNoise Generation for Digital Signal Synthesis", IEEE Trans. Instrumentation and Measurement, Vol. IM-35, No. 4, Dec. 1986). However, with such an approach, if processing is done in real time the noise bandwidth is limited by the processor speed. If random-stored values are used, the requirement for some values to occur with low probability makes the memory size prohibitive.